Programmable analog circuits are often required in applications where voltage accuracy and low power use are desirable traits.
Band gap reference voltage circuits are frequently used in applications that require a high degree of voltage accuracy. Band gap voltage reference circuits are known for their capabilities for providing excellent accuracy and stability over time and a range of operating temperatures. Unfortunately, however, band gap references are limited to a fixed voltage level, typically about 1.2V. The additional circuitry required for providing other voltage levels, such as fixed gain amplifiers for example, can be seriously detrimental to accuracy. Additionally, band gap voltage reference circuits generally draw a significant amount of power, presenting an additional problem in applications in which low power consumption is desirable.
Floating gate voltage reference circuits are often chosen for their low power requirements, but can be problematic in applications requiring a high degree of accuracy in providing a selected programmed voltage level, particularly over time and changes in temperature. A floating gate may be conceptualized as a charge oasis of conductive material electrically isolated from the outside world by a semiconductor substrate desert. The floating gate is capacitively coupled to the substrate or to other conductive layers. The floating gate is usually used to provide bias to the gate of a transistor and is readable without causing a significant leakage of charge. In theory, a floating gate programmed at a particular charge level remains at that level permanently, since the floating gate is insulated by the surrounding material. The floating gate is commonly charged using Fowler-Nordheim tunneling, or Channel Hot Carrier (CHC) tunneling, practices generally known to practitioners of the microelectronic arts. The accuracy of common floating gate circuits is limited for at least two primary reasons. Firstly, the potential on a floating gate decreases after it is programmed due to the capacitance inherent in the tunneling device. This voltage offset is well-defined and predictable, but is unavoidable in prior art floating gate voltage reference circuits because the capacitance of the tunneling device cannot be completely eliminated. Secondly, the accuracy of prior art floating gate voltage reference circuits is also hampered by the decay of the theoretically permanent charge on the floating gate over time. The decay of the charge over time occurs due to various factors, including the gradual escape of electrons from the tunneling device, and dielectric relaxation of the floating gate capacitors. The decay of charge is not entirely predictable since it can be influenced by environmental factors such as mechanical and thermal stress effects or other variables.
Due to these and other problems and potential problems, improved floating gate reference and feedback circuits would be useful and advantageous in the arts. Floating gate circuit architecture and systems adapted to rapid and accurate offset compensation would be particularly beneficial contributions to the art.